Recently, in order to cope with global warming, power generation using natural energy (so-called renewable energy), such as solar power generation or wind power generation, has been actively conducted worldwide. The output of such power generation depends largely on natural conditions, such as weather. Accordingly, it is expected that when the percentage of natural energy used in electric power systems increases, problems may arise during the operation of electric power systems, such as difficulties in maintaining frequencies and voltages. As a countermeasure for such problems, it is considered to install large-capacity storage batteries so that smoothing of variations in output, storage of surplus electricity, leveling of loads, and the like can be achieved.
A redox flow battery shown in an operating principle diagram of FIG. 4 (hereinafter referred to as the RF battery β) is a large-capacity storage battery. The RF battery β, which is typically connected, through an AC/DC converter, between a power generation unit (such as a solar photovoltaic power generator, a wind power generator, or a general power plant) and a load (such as a consumer), charges and stores electricity generated by the power generation unit, or discharges and supplies stored electricity to the load.
The RF battery β includes a single or a plurality of battery units 100. A battery unit 100 includes a positive electrode cell 102 that contains a positive electrode 104, a negative electrode cell 103 that contains a negative electrode 105, a membrane 101 that separates the cells 102 and 103 and permeates ions, and performs charging and discharging. The positive electrode cell 102 is connected via ducts 108 and 110 to a positive electrode electrolyte tank 106 that stores a positive electrode electrolyte. The negative electrode cell 103 is connected via ducts 109 and 111 to a negative electrode electrolyte tank 107 that stores a negative electrode electrolyte. Furthermore, the ducts 108 and 109 are provided with pumps 112 and 113 that circulate their corresponding electrolytes, respectively. In the battery unit 100, the positive electrode electrolyte in the positive electrode tank 106 and the negative electrode electrolyte in the negative electrode tank 107 are circulated and supplied to the positive electrode cell 102 (positive electrode 104) and the negative electrode cell 103 (negative electrode 105), respectively, by ducts 108 to 111 and the pumps 112 and 113, and charging and discharging are performed in response to changes in the valence of metal ions (vanadium ions in the example shown) serving as active materials in the electrolytes at the two electrodes.
In the RF battery β, the gas phase inside each of the electrolyte tanks 106 and 107 expands or contracts when subjected to temperature changes in the installation environment, heat generation during charging and discharging, or the like. For example, when the pressure inside each of the electrolyte tanks 106 and 107 becomes a positive pressure greater than the atmospheric pressure, there is a concern that each of the electrolyte tanks 106 and 107 may explode. Furthermore, when the pressure inside each of the electrolyte tanks 106 and 107 becomes a negative pressure less than the atmospheric pressure, there is a concern that each of the electrolyte tanks 106 and 107 may be compressed and damaged. As a countermeasure for this problem, it has been proposed to provide a pressure adjustment mechanism on the redox flow battery, the pressure adjustment mechanism being configured to adjust the pressure inside each of the electrolyte tanks 106 and 107 to approximately the atmospheric pressure (for example, refer to PTL 1).
PTL 1 discloses a pressure adjustment mechanism including a first atmospheric pressure-maintaining container and a second atmospheric pressure-maintaining container in which a pressure adjusting liquid is stored (refer to FIG. 1 of PTL 1). The gas phase inside the first atmospheric pressure-maintaining container is made to communicate with the gas phase inside a liquid storage tank (electrolyte tank) by a first communicating means, and the liquid phase inside the first atmospheric pressure-maintaining container and the liquid phase inside the second atmospheric pressure-maintaining container are made to communicate with each other by a second communicating means. Furthermore, the gas phase inside the second atmospheric pressure-maintaining container is open to the atmosphere. In the pressure adjustment mechanism having such a configuration, when the pressure inside the electrolyte tank becomes positive, as shown in FIG. 2 of PTL 1, the liquid surface of the first atmospheric pressure-maintaining container falls, and the liquid surface of the second atmospheric pressure-maintaining container rises, thereby decreasing the pressure inside the electrolyte tank to approximately the atmospheric pressure. On the other hand, when the pressure inside the electrolyte tank becomes negative, as shown in FIG. 3 of PTL 1, the liquid surface of the first atmospheric pressure-maintaining container rises, and the liquid surface of the second atmospheric pressure-maintaining container falls, thereby increasing the pressure inside the electrolyte tank to approximately the atmospheric pressure.
Furthermore, in the configuration of PTL 1, when the pressure inside the electrolyte tank becomes negative, the atmosphere can be prevented from being sucked into the electrolyte tank, and it is possible to prevent degradation of the electrolyte due to the atmosphere. The atmosphere can be prevented from being sucked into the electrolyte tank because the second communicating means that connects the second atmospheric pressure-maintaining container and the first atmospheric pressure-maintaining container to each other is filled with the pressure adjusting liquid.